After we turned on streaming for our assistant, tool calls failed with JSON parse errors and sometimes ran with another call's arguments. We were parsing argument fragments per chunk and keying them by arrival order. Accumulating by index fixed it.
One CI run in eight failed somewhere in our checkout E2E suite, never in the same test twice. The causes were fixed sleeps, URL checks that raced navigation and text selectors that matched two buttons. Web-first assertions, role locators and traces fixed it.
Our PDF invoices and the amounts charged through Stripe disagreed by a cent on 214 invoices in one month. Floats, toFixed and rounding at every step were to blame. We moved to integer minor units with one explicit rounding point.
A billing test passed for weeks and failed this morning, 31 January, for every engineer at once. JavaScript's setMonth rolled 31 January into 3 March, and the test read the real clock. We fixed the arithmetic, injected the clock and added property tests over every day.
After upgrading to Next.js 15 our dev console filled with sync dynamic API warnings, and one un-awaited auth helper quietly stopped redirecting logged-out users. How we migrated cookies(), headers() and params safely.